Continuity mistake: When Melanie reaches for the flashlight on the coffee table it's been moved closer to and slightly in front of the kerosene lamp and she hasn't touched it. (01:45:20)
Continuity mistake: Two seconds after the orange car explodes quite a few people appear who were nowhere nearby a second earlier. (01:25:35)
Continuity mistake: As Mitch returns to the living room before the lights go out, there's much more wood in the pile than before when he was putting it into the fire. (01:44:05)
Continuity mistake: As Melanie leaves Mitch's house, the barn alternates between having a blue sky behind it and having clouds behind it. (00:23:00)
Continuity mistake: No one touches the two yellow buoys on the end of Mitch's doc, but after Melanie leaves the birds in the house they've changed position relative to the wooden planks underneath. (00:23:10)
Continuity mistake: Melanie hasn't been near her car since the attack on the children at the school. Now as she and Mitch pass it while walking to Annie's, we can see the front wheels have been turned all the way to the right. (01:29:50)
Continuity mistake: As Melanie watches Mitch from out on the boat, someone opened the shades on the left lower window of the house. In the next shot as Mitch watches her with the binoculars the shades are closed again. (00:24:10)
Continuity mistake: When the store manager shows Melanie Mitch's house, the cloud cover is different from what we just saw behind her car as she drove up to the store. (00:13:20)
Continuity mistake: When Melanie first reaches into her pocketbook for the card she's about a foot away from the cage. In the next shot as she puts the card against the cage her pocketbook is practically touching the seat. (00:22:25)
Continuity mistake: After dinner with Mitch, Melanie walks into Annie's apartment and between shots the shadow of Annie's head appears out of nowhere against the wall. (00:41:05)
Continuity mistake: Before Melanie sits down at the table the position of the metal milk container changes from the left to the right side of the table between shots. (00:26:45)
Continuity mistake: At the end of the bird attack Melanie gets Lydia and Cathy out through the side door. If you look to the left of the screen the wall behind and around the lamp where the painting hangs is dark. In the last shot of the birds flying around the painting is lit. (00:56:00)
Continuity mistake: As Melanie walks up to Annie's, the porch casts it's shadow on the front door. In the next shot the door is lit up by sunlight. (00:16:20)
Continuity mistake: The lighting has changed as Melanie turns around to talk to Mitch's neighbor. When she first turns around her shadow is not on the wall and in the next shot it is. (00:10:25)
Continuity mistake: When we first see Melanie driving up to Bodega Bay there's no windshield in her car. (00:11:00)
Continuity mistake: As Annie goes across the room to get an ashtray, you can see that the next room is pitch black. In a previous shot when she went to get the brandy it was lit up and she never touched a light switch. (00:44:10)
Continuity mistake: As Mitch runs to get the hammer and nails for the front door notice the top leaf of the fern plant to his left. It gets rearranged in the next shot, as Melanie watches him, with one fern sticking out from the middle that wasn't there before. (01:43:45)
Continuity mistake: As Melanie starts off across the Bay, we see two boats and a red building to the left of the matte drawing that weren't there when she approached the boat keeper. (00:20:45)
Continuity mistake: When Annie gets up and says, " I think I'll have some of that" her cigarette is half gone with a long ash on it. In the next shot as she fiddles with the brandy bottle her cigarette is brand new. (00:42:50)
Continuity mistake: When Mitch picks up the mirror his right hand grabs one of the umbrellas hanging on the right of the mirror. It in the next shot the umbrella has disappeared. (01:43:30)
Answer: It is never explained, probably because it would ruin the mystery of the film to spell it out.
If you are a Hitchcock fan, you pay attention to dialogue. They mention quite often that it started Melanie's arrival, of which she brought 2 caged birds. The rest is implied.